When assigning a product to a category, the primary and classification categories are used to determine what information appears for the product in the storefront.
The classification and primary categories are also used in SEO to determine the URL and the sitemap attributes used for the product.
Each product has one classification category and primary category. However, products can be assigned to other categories. If the categories are in the storefront catalog, the products are available for sale in those categories.
The primary category you select for an item determines the breadcrumb for the search result of the product or content detail page. If no primary category is set for a product, the classification is used to determine the category links. This breadcrumb remains the same, no matter what category you view the product in.
For example, if a customer searches for a man's boot, you want the breadcrumb for the result to point to a narrow category such as men’s boots. You don’t want the breadcrume to pint to a broad category, such as shoes, or one that isn't applicable, such as women's shoes. The classification you select determines the attributes shown for the result. If no classification value is set, the attribute value is taken from the primary category.